1. Field of the Invention
The invention is in the field of monitoring optical fibre networks. More particularly, it is in the field of monitoring the physical state of optical fibre connections by means of fault location techniques with the aid of optical time-domain reflectometry (OTDR) based on correlation. The invention relates to an application of such a fault location technique in a branched network of an optical fibre connection during operation.
2. Prior Art
To monitor the physical state not only of a single optical fibre connection but also of branched optical fibre networks, it is generally known to apply OTDR techniques for locating faults such as optical fibre rupture. In this technique, a short optical pulse is launched at an input end of the optical fibre connection or the optical fibre network and the reflected optical signal is then detected at the same input end during a certain period of time. Since near reflections are back earlier at the input end than those further away, the reflected power in the optical fibre connection or the optical fibre network can be measured as a function of the distance from the input end. Not only do strong reflections occur as a consequence of the presence of connectors or mechanical splices, but weak reflections are also present as a consequence of Rayleigh backscattering. Optical fibre rupture and its location can easily be detected since a signal is no longer reflected from distances beyond a rupture. Reference [1] discloses a method and a device for using OTDR in which, instead of a short optical pulse, series of optical pulses are injected into the optical transmission medium, such as an optical fibre and in which the reflected signal is then correlated with the injected signal. From the correlation, a signal is obtained which represents the amplitude of the reflected signal as a function of the time which has elapsed since the pulse series were injected or as a function of the distance traversed from the point at which the pulse series were injected. The disadvantage of such an OTDR technique is that the launching of an additional signal is required. Since it is preferably precisely during operation that monitoring of the connection or of the network takes place, special procedures are necessary in order not to disturb the signal traffic passing via the connection or the network. As a solution to this problem, it is known, for example from reference [2], to use a wavelength of the pulse signal for fault location which is markedly different from the communication wavelength. A solution in which no additional signal is launched is known from reference [3]. In an optical communication system, a number of bidirectional optical fibre lines connect an exchange to an equal number of subscribers. Both in the exchange and in the case of each subscriber, an optical transmitter and an optical receiver are connected to the end of the optical fibre line concerned. As a consequence of an additional reflection means, each optical receiver at the subscriber end continuously reflects a part (preferably 15%) of the received optical signal into the optical fibre line back to the exchange. In the exchange, the reflected signal of each subscriber is detected during a short time consecutively from each optical fibre line. If the detected reflection signal of an optical fibre line is above a preset threshold value which is matched to the reception of the signal additionally reflected in each receiver, switching takes place to the next optical fibre line. If the detected reflection signal is below the threshold value, then, in addition to an alarm signal, a signal is also emitted which switches on a correlator device in which the detected reflection signal is correlated with a delayed electrical signal corresponding to the optical signal transmitted to the subscriber concerned. As a result of the correlation, a signal is obtained for optical time-domain reflectometry of the optical fibre connection to the subscriber concerned. This known method of performing OTDR based on correlation is hereinafter referred to as correlation OTDR. The great advantage of such a monitoring system based on correlation OTDR is that, for the purpose of correlation, use is made of a signal which has in any case to be transmitted by the exchange to the subscriber concerned and it is therefore not necessary to launch any additional signal which may also have a disturbing effect on the communication. Disadvantages of this known technique are, however, that this method of monitoring is tied to an end of an optical fibre connection and that an intervention in the transmitting equipment is also required by the method in which the signal used for the correlation is obtained.